Associative properties of the perirhinal network.

The perirhinal area is a rostrocaudally oriented cortical region involved in recognition and associative memory. It receives topographically organized transverse projections from high-order neocortical areas and is endowed with intrinsic longitudinal connections that distribute neocortical inputs rostrocaudally. Earlier work has revealed that neocortical inputs strongly recruit perirhinal interneurons located at the same transverse level, limiting the depolarization of principal cells. In contrast, at a distance, neocortical stimuli only evoke excitation because longitudinal perirhinal pathways do not engage interneurons. This raises the possibility that the perirhinal network allows for Hebbian-like associative interactions between coincident and spatially distributed inputs. To test this, we analyzed the effects of theta-frequency neocortical stimulation using simultaneous field potential recordings and optical imaging in the whole guinea pig brain in vitro. Theta-frequency stimulation (TFS) at one neocortical site resulted in a prolonged input-specific response depression at all perirhinal levels. In contrast, paired TFS of 2 distant neocortical sites resulted in a prolonged response potentiation to the paired inputs, suggesting that longitudinal perirhinal connections can support associative interactions between coincident but spatially distributed inputs. Moreover, we found that induction of these 2 forms of plasticity depended on the competing influence of glutamate group I metabotropic and NMDA receptors, respectively.